Depression is defined by feelings of sadness and hopelessness, not related to life events, that interfere with the performance of daily life activities. Unfortunately, depression is becoming more prevalent. It is estimated that 11.2% of the Canadian population suffers from depression at some time in their lives (1) and it is predicted that depression will soon be second only to heart disease as the leading cause of healthy years of life lost worldwide (2).
PREVIOUS STUDIES ON DEPRESSION
Existing evidence from the past couple of decades has uncovered a relationship between the types of foods consumed and the incidence of depression. Healthy diets have been linked in many studies to positive mental outlook. For example, a 2009 study showed that those eating more whole foods have fewer symptoms of depression than people eating more processed foods (3). The year 2010 brought a similar study, illustrating that Japanese citizens eating the highest amounts of vegetables, fruit, mushrooms and soy products suffered 60% fewer depressive symptoms than those eating the lowest amounts of these foods (4). More recently, a 2018 review of 41 previous studies showed that a diet rich in fruits, vegetables, whole grains and legumes and avoiding processed meat, dairy and trans-fats could reduce risk of depression by 35% while diets high in processed meats and trans-fats increased the risk of depression (5). All these trials and others too indicate that dietary choices may have a substantial effect on the development of depression. But many of these studies were observational or epidemiological in nature in which the mood of the subjects was simply compared with the content of their diets to see if any links between the two could be discovered. The problem with observational research is that, although it can point out an association between two factors, it cannot establish cause and effect. When it comes to the results of observational studies on depression, questions arise. Are depressed people simply more likely to eat poorly due to their depression or is it the unhealthy diet that brings on the depression?
THE ADVANTAGE OF RANDOMIZED CONTROLLED TRIALS
Randomized controlled trials are regarded as the gold standard of scientific studies. A properly crafted randomized controlled study has the ability to demonstrate that a specific intervention causes or doesn’t cause a certain result. In such studies a change is made in one group of subjects (the intervention group) and its consequences are compared to those of another group (the control group) who do not experience the change being studied. Control groups can be either inactive or active. In dietary studies on depression, inactive control groups are those in which participants maintain their habitual diets and receive no additional intervention during the trial period; active control groups receive only benign interventions such as social support or counselling with the amount of time and attention spent on each study participant matched in the intervention and control groups.
A RECENT RANDOMIZED CONTROLLED STUDY ON DIET AND DEPRESSION
Randomized controlled depression studies have indeed already been completed and, in 2019, a research team in Australia performed a meta-analysis on sixteen such studies whose results looked at the relationship between diet and depression (6). A meta-analysis is a re-analysis of the results of all the included studies so that the overall combined effect of an intervention can be revealed. This particular meta-analysis included studies using either active or inactive control groups and included almost 46,000 subjects suffering from both clinical and sub-clinical depression. Eleven of the trials also examined the effect of diet on anxiety symptoms.
Each of the sixteen studies included in this meta-analysis compared the effect of whole diet interventions on depression to a control group receiving no dietary intervention. Interventions focusing on single food components were not eligible. Changes in depressive symptoms were assessed using accepted depression scores such as the “Hamilton Rating Scale for Depression” and the “Beck Depression Inventory”.
What Does This Meta-Analysis Tell us?
Results of this meta-analysis reveal clearly that healthier dietary patterns produce significant and consistent positive effects on depressive symptoms compared to less healthy dietary patterns. Additionally, the positive association between healthy diets and depression was independent of body weight.
Healthier dietary patterns included in this meta-analysis;
- Increasing high-fiber nutrient-dense whole foods such as vegetables, fruits, whole grains, nuts and seeds
- Reducing high-fat foods, high-sugar foods, processed foods and most animal products
- Reducing body weight
No advantageous effects on anxiety were observed in this meta-analysis.
What is Causing These Beneficial Effects?
The researchers point out that the mechanisms for these improvements in depression scores are not yet fully understood. However, there are several physiological pathways that might hold the answers to this question.
The acknowledged cause of depression for many years has been that it is an imbalance of chemical neurotransmitters such as serotonin in the brain. There are a couple of conundrums with this idea. First of all, antidepressants are effective at increasing serotonin levels almost as soon as they are started but it takes several weeks to see any therapeutic effect on the depression itself. Additionally, antidepressants are only successful in about 50% of depression patients (7).
Emerging research is offering new insights into the possible causes of depression.
Oxidative Stress and Inflammation
Research shows that people with depression have 46% higher levels of C-reactive protein, a marker for inflammation (8).
People suffering from depression also display higher levels of oxidative stress. Oxidative stress occurs in all cells as they perform their functions. The result of oxidative stress is the production of free radicals including reactive oxygen species (ROS), reactive nitrogen species (RNS) and other free radicals. Free radicals are highly reactive molecules. Antioxidants supply the extra electrons required to neutralize free radicals. If there is a higher level of pro-oxidants compared to antioxidants, damage from oxidation and increased inflammation are the results (9).
The brain is especially sensitive to oxidative damage. In people with depression, increased levels of ROS and RNS along with deficient antioxidant defenses such as lower levels of vitamin E, zinc and coenzyme Q10 have been demonstrated through post-mortem examinations of the brains from people who had suffered from major depressive disorder (9).
How can we increase the level of antioxidants in our body? Antioxidant-rich foods originate from the plant kingdom while animal-based foods are low in antioxidants. There are thousands of bioactive antioxidant phytochemicals found in plants. The answer here is easy. Eat plants! (10)
Reduced production of new brain cells
It was once believed that the brain was incapable of producing new cells after birth. There is now convincing evidence that at least two regions in the brain continuously generate new neurons. One of these is the hippocampus, a part of the brain believed to be involved in mood disorders.
There are a couple of dietary factors that participate in the production of new brain cells and may play a role in lessening depression.
There is a growing body of data from both animal and human studies illustrating that dietary polyphenols reduce anxiety and have antidepressant-like effects. This may be due in part to their anti-inflammatory influences, antioxidant properties, prebiotic effects or neuroprotective traits. But it may also be the result of the involvement of polyphenols in the regulation of new cell production in the brain (11)
Research has noted that depression is associated with decreased generation of new cells in the hippocampus, a region of the brain that regulates emotion, mood, cognition and stress. Recent investigations have given rise to new theories suggesting that stress may be causing reduced brain cell production in the hippocampus and could in fact be a causal factor in depression. Patients with depression indeed show decreased hippocampal volume (7).
The production of new cells is metabolically demanding and requires a lot of energy. It is hypothesized that a lack of energy may be at the root of reduced brain cell generation. Mitochondria are small organelles present within most cells of the body that generate the energy needed for the cells to function. Dysfunction of mitochondria appears to result in a lack of the energy required to produce new cells. This energy deficit is now being proposed as a cause of depression. The brain is a highly active organ, requiring about twenty times more energy by weight than elsewhere in the body, making it very susceptible to reductions in energy supply (7).
The origin of the stress causing the decline in new cell production may be a deficiency in the nutrients required to keep the mitochondria in tip-top working order. Healthy whole plant foods including colourful vegetables and fruit, whole grains and nuts provide both a rich source of polyphenols as well as an abundance of the nutrients required for optimal mitochondrial function (12).
Gut Microbiota Dysbiosis
The microbiome is involved in the body’s response to stress, the formation of new nerve and brain cells and the communication between these cells. A strong link has been identified between the gut microbiome and brain development and behavior, including the development of neuropsychiatric disorders such as depression, anxiety and cognition.
Animal studies have shown that changes in the composition of the microbiome affect symptoms of anxiety and depression. In humans, gut microbiota from the Bacteroides family are associated with depression. Chronic low-level inflammation such as that associated with increased gut permeability (leaky gut syndrome) seems to be a hallmark of depression (13,14) .
The optimal diet for a healthy gut microbiome is a whole-food plant-based diet.
Too many Unhealthy Foods in the Diet
Eating high levels of detrimental inflammation-causing foods such as foods high in saturated and trans-fats, processed meats, artificial sweeteners and refined carbohydrates is associated with increased depression risk.
Increasing plant-based foods in the diet leaves less room in the stomach for harmful foods.
Overweight and Obesity
Reducing extra weight can benefit physiological well-being in many ways. Epidemiological studies show a robust link between being overweight and elevated risk of depression. In studies, individuals who lose the most weight show the greatest improvements in measures of well-being (15).
Inflammation is a core feature of depression. Excess fatty tissue increases the production of pro-inflammatory compounds such as cytokines. Recent research has also shown that dietary-induced obesity reduces insulin signaling in the brain, thereby increasing brain inflammation (16).
THOUGHTS FOR THE FUTURE
Factors that determine good mental health are complex. However, there are strong associations between a poor diet and worsening of mood disorders. On the other hand, improving diets has revealed consistent benefits for both physical and mental health. For example, increased consumption of a diet rich in fresh fruits and vegetables has been associated with increased reported happiness and higher levels of mental health and well-being (17). Best of all, healthy changes in diets have no harmful side effects.
All these factors make dietary improvement an ideal first treatment for depression or for self-management for those suffering from sub-clinical depression. Additionally, healthy diets show extra benefits when combined with other strategies against depression such as exercise or when added to anti-depressant therapy (14).
No trials have yet directly compared the effectiveness of dietary interventions to antidepressant medications. It will be interesting to see the results of such studies.
SUMMING IT ALL UP
A whole-food plant-based diet provides all the nutrients required to enhance mental health.
Antioxidant compounds, polyphenols and fiber found only in plant foods reduce oxidative stress and lower inflammation, nourish mitochondria, support healthy body weight and foster a diverse and robust gut microbiome.
1 Knoll, A D., MacLennan, R.N. Prevalence and correlates of depression in Canada: Findings from the Canadian Community Health Survey. Canadian Psychology 2017; 58(2), 116-123.
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4 Nanri, A., Kimura, Y., Matsushita, Y., Ohta, M., Sato, M., Mishima, N., Sasaki, S., Mizoue, T. Dietary patterns and depressive symptoms among Japanese men and women. Eur J Clin Nutr. 2010 Aug; 64(8):832-839.
5 Lassale, C., Batty, G.D., Baghdadli, A., Jacka, F., Sánchez-Villegas, A., Kivimäki, M., Akbaraly, T. Healthy dietary indices and risk of depressive outcomes: a systematic review and meta-analysis of observational studies. Mol Psychiatry. 2018 Sep 26. doi: 10.1038/s41380-018-0237-8.
6 Firth, J., Marx, W., Dash, S., Carney, R., Teasdale, S.B. et al. The effects of dietary improvement on symptoms of depression and anxiety: a meta-analysis of randomized controlled trials. Psychosom Med. 2019 Feb 5. doi: 10.1097/PSY.0000000000000673.
7 Allen, J., Romay-Tallon, R., Brymer, K.J>, Caruncho, H.J., Kalynchuk, L.E. Mitochondria and Mood: Mitochondrial Dysfunction as a Key Player in the Manifestation of Depression. Frontiers in Neuroscience, 2018; 12 DOI: 10.3389/fnins.2018.00386
8 Soledad Cepeda, M., Stang, P., & Makadia R. Depression Is Associated With High Levels of C-Reactive Protein and Low Levels of Fractional Exhaled Nitric Oxide: Results From the 2007-2012 National Health and Nutrition Examination Surveys. J Clin Psychiatry. 2016: 1666-1671.
9 Salim, S. Oxidative stress and psychological disorders. Curr Neuropharmacol. 2014;12(2):140–147.
10 Carlsen, M.H., Halvorsen, B.L. et al. The total antioxidant content of more than 3100 foods, beverages, spices, herbs and supplements used worldwide. . Nutr. J. 2010; 9:3.
11 Dias, G.P., Cavegn, N., Nix, A., et al. The role of dietary polyphenols on adult hippocampal neurogenesis: molecular mechanisms and behavioural effects on depression and anxiety. Oxid Med Cell Longev. 2012;2012:541971.
13 Evrensel, A., Ceylan, M.E. The Gut-Brain Axis: The Missing Link in Depression. Clin Psychopharmacol Neurosci. 2015;13(3):239–244.
14 Adan, R.A.H., van der Beek, E.M., Buitelaar, J.K., Cryan, J.F., et al. Nutritional psychiatry: Towards improving mental health by what you eat. European Neuropsychopharmacology. Dec 2019; 29(12): 1321-1332.
15 Swencionis, C., Wylie-Rosett, J., Lent, M.R., et al. Weight change, psychological well-being, and vitality in adults participating in a cognitive-behavioral weight loss program. Health Psychol. 2013;32(4):439–446.
16 Melo, H.M., Santos, L.E., Ferreira, S.T. Diet-Derived Fatty Acids, Brain Inflammation, and Mental Health. Front Neurosci. 2019;13:265. Published 2019 Mar 26.
17 Conner, T.S., Brookie, K.L., Carr, A.C., Mainvil, L.A., Vissers, M.C. Let them eat fruit! The effect of fruit and vegetable consumption on psychological well-being in young adults: A randomized controlled trial. PLoS One. 2017 Feb 3;12(2):e0171206.